Add superproject directive to studio-master-dev manifest

This makes it possible to use "repo init --use-superproject" to
work on Android Studio using the superproject at
https://cs.android.com/android/platform/superproject/+/studio-master-dev:;bpv=1
to determine which revisions to sync to (speeding up syncs).

Bug: 179696378
Change-Id: Ibcef12a02f980366250b9edd09461e051ae08294